Its never a good idea to mix ages, but its done all of the time. note the many problems written about and discussed on the internet. There is too much Calcium in layer feed for growing young birds.

What is the the ideal mix for pullets? Our feed mill said to feed layer mix to all of them. Ours are separated but we are feeding all of them layer mix. The layer mix includes soy and corn. They seem very healthy.

Its best to use starter or starter/grower until lay begins.Then mix the last of the starter with the lay feed rather than just simply switch feeds all at 1 time. Most any of the commercial feeds are adequate.
Feed mill guys generally try to be helpful but few have animal nutrition backgrounds.

Thank you, Fancifowl. Do you know how "broiler starter" and regular starter are different? These are regular breed chickens, for laying and meat.